1. Field of the Invention
The present invention relates to an image recording apparatus, and particularly to an image recording apparatus in which a laser beam modulated by an image signal is emitted from a light source such as a semiconductor laser or the like and is used for optically scanning the surface of a recording medium through a deflecting unit such as a rotating polygon mirror or the like to record an image thereon, and which is preferably applied to laser beam printers, facsimile and the like. The image recording apparatus has a variable pixel density in the subscanning direction.
2. Related Background Art
Various image recording apparatuses have previously been proposed in which an image is recorded by optically scanning the surface of a recording medium using the laser beam emitted from a laser source and modulated on the basis of image information through a deflecting unit such as a rotating polygon mirror or the like and a lens system having f.theta. characteristics.
A laser beam printer of such image recording apparatuses has the characteristics that the noise produced during the operation is low due to its non-impact operation and that high-definition (high-resolution) printing can be relatively easily performed.
In such a laser beam printer, the surface of a recording medium generally has a constant dot density. An example of methods of making the dot density on the surface of the recording medium variable is a method of making the rotation number of a rotating polygon mirror or the speed (rotational speed) of the recording medium in the subscanning direction variable in view of the structure of the laser scanning optical system.
The method of making the rotational number of the rotating polygon mirror variable for making the dot density of image information variable causes deterioration in the basic driving state of a motor used as a driving means. Particularly, the method has the problems that it is difficult to precisely control the changing amount of the rotation number and attempt to stabilize the motor without producing jitters or vibrations when the rotation number is changed.
There is also the problem that the current cannot be set to a minimum value because the rotation number of the motor strictly varies.
In addition, since the rotation number of the motor is within a range optimum for setting, the variable range of the rotation number is within about 50% of the basic rotation number.
Namely, the method of making the rotation number of the polygon mirror variable has difficulty in changing the dot density over a wide range and causes deterioration in quality of the image printed due to the instability of the motor even when the dot density is changed within a narrow range.
Further, for example, when the dot density is doubled both in the main scanning and subscanning directions, the sending speed of the image signal generated from an image signal generating section must be quadrupled. Even when the dot density is changed only in the subscanning direction, the sending speed of the image signal generated from the image signal generating section must be changed. The load on the image signal generating section is thus increased.
On the other hand, the method of making the speed of the recording medium variable causes the need for changing the process conditions of an apparatus which employs an electrophotographic process according to the speed of the recording medium. Since the electrophotographic process includes many units, it is difficult to variably control all the units according to the speed of the recording medium, and this method is thus impossible to attempt to improve image quality.
None of the above methods is thus capable of partially changing the dot density of an image on one page when the electrophotographic process is employed. In addition, although the power of the laser beam emitted from a semiconductor laser must be variably controlled over a wide range according to at least the dot density, this variable control is difficult from the viewpoint of variations in the emission angles of semiconductor laser solids, and the dot density cannot be changed over a wide range.
On the other hand, there has recently been a demand for recording various types of image information by a single laser beam printer. For example, when a half tone is recorded with a high gradation, or when a line image is recorded with higher definition (higher resolution), it is indispensable to change the dot density over a wide range.